This invention relates to a method and apparatus for electronically determining the acceptability of a coin. More particularly, this invention relates to an apparatus that determines the validity and value of a coin as the coin passes through an electromagnetic field.
In the prior art it is known to insert coins through a coin detection device having an inductive electromagnetic field. The effect upon field variation as the coin passes through the electromagnetic field is detected by sensing frequency and/or amplitude changes of an oscillating electrical signal through the inductors. Characteristics of the oscillating signals are then compared with data stored in a programmable memory. If these characteristics are within the predetermined limits for acceptable coins of a given denomination, then the apparatus indicates that the coin is acceptable. Examples of devices which measure coins and determine whether they are within predetermined limits of an acceptable value are disclosed in U.S. Pat. Nos. 3,918,565 to Fougere et al. and 3,653,481 to Boxall et al.
A drawback to the devices disclosed in the aforementioned patents is that only certain physical characteristics of the coin, such as the coin's diameter or thickness, may be detected by the device. When a coin is bent or chipped many of the characteristics of the oscillating signals for the coin being detected could be inaccurate. Thus, when the detection device detects these inaccuracies the coin could wrongfully be rejected. Further, if the parameters being detected are of a fraudulent coin, generally referred to as a "slug", have the same characteristics as an acceptable coin, the "slug" could be accepted by the device.
Techniques for overcoming the aforementioned limitations are disclosed in U.S. Pat. Nos. 4,353,452 to Shah et al., 4,742,903 to Trummer and 4,754,862 to Rawicz-Szczerbo et al. These patents disclose a technique of feeding a coin successively and simultaneously through multiple high-frequency test signals to obtain more than one characteristic of the coin. However, certain characteristics of the coin may still not be detected, and a slug could wrongfully be accepted.
Techniques have been proposed for measuring more than one 15 parameter of a moving coin. An example of such a technique is disclosed in U.S. Pat. No. 4,488,116 to Plesko. This technique places a coin between two different electromagnetic fields and then measures the coin's interaction on the field. However, this technique may not be able to detect all the characteristics of the coin, allowing the possibility of a slug being accepted.
Other drawbacks to prior art coin detection devices is that when the characteristics of the oscillating signal being measured are close to the characteristics of the oscillating signal for the coins that are acceptable, the coin-detecting device may not be able to distinguish an acceptable coin from an unacceptable coin. Further, the coin detection device's sensitivity may change with aging or with room temperature changes. Consequently, the detection of the characteristics of the oscillating signal could become inaccurate resulting in an increase in the uncertainty of coin detection.
Most of the aforementioned techniques convert the detected characteristic into a numeric value and then preselect a value or a range of values for each coin which is acceptable. If the numeric value of the characteristics of the oscillating signal falls outside this range, the coin will be rejected. Thus an otherwise acceptable coin may be rejected, where one physical characteristic does not meet an acceptable limit.
Due to aging and temperature variations of the coin detection device, it may be necessary to have the device calibrated. Examples of calibration techniques are disclosed in U.S. Pat. No. 4,471,864 to Marshall and Great Britain 024,398. Great Britain 024,398 discloses a technique for calibrating a range of values for each coin which allows the coin to be accepted. However, these calibration techniques set fixed limits on the range of values in which the coin's characteristics must be within. Consequently, a valid coin that has values that are close to the range, but outside the limits, may be inadvertently rejected.
Another technique for calibrating the coin detection device is disclosed in U.S. Pat. No. 4,471,864. This device uses a reference oscillator which is continuously in operation. The reference oscillator generates correcting signals which are fed back to a main oscillator to maintain the main oscillating output at a constant amplitude. This calibration circuit provides correcting signals to maintain the output for each coin at a constant repeatable value. However, if the oscillator circuits vary due to aging, the circuit could generate inaccurate outputs. Thus, this coin detector could provide a faulty indication.